1. Field of the Invention
The present invention relates to an automatic sampler for automatically injecting a sample into an analytical flow path of a liquid chromatograph.
2. Description of Background Technique
A common automatic sampler for automatically injecting a sample into an analytical flow path of a liquid chromatograph is formed to take in a predetermined amount of a sample with a metering pump with a needle inserted in a sample vessel and to inject the taken-in sample into the liquid chromatograph (see Japanese Patent Application Laid-Open No. 2005-99056, for example).
FIG. 10 shows an example of a prior-art automatic sampler with a liquid chromatograph.
In this automatic sampler, by switching a first switching valve 8 and a second switching valve 18 and driving a needle 20, flow paths for carrying out a sample intake step, a step of introducing the sample into the liquid chromatograph, a step of cleaning the needle 20, a step of discharging a rinse solution from a cleaning port 16, and a rinse solution addition step are formed.
For example, at the time of the intake of the sample, with the needle 20 inserted into a sample vessel 15, a central common port connected to an intake port of a metering pump 5 and a port “A” are connected in the first switching valve 8 and ports “a” and “b” are connected in the second switching valve 18. As the metering pump 5, a syringe pump is used. In this state, by driving a plunger of the metering pump 5 toward a suction side, the sample is taken in from a tip end of the needle 20 and stays in a sample loop 22 provided on the flow path for connecting the port “a” of the second switching valve 18 and the needle.
At the time of the addition of a rinse solution to the cleaning port 16, after the metering pump 5 takes in any of a rinse solution “A”, a rinse solution “B”, and a mobile phase “C”, the central common port connected to the metering pump 5 and a port “F” are connected in the first switching valve 8, and the plunger of the metering pump 5 is driven toward a discharge side, and any of the solutions is discharged to the cleaning port 16.
In the automatic sampler in FIG. 10, whether an amount of the solution, such as the sample to be injected into the liquid chromatograph, is large or small, the single metering pump 5 meters and takes in the sample. Therefore, if a metering pump 5 of a small cylinder capacity is used, an amount of the liquid which can be treated by a stroke of driving of the plunger is small. If an intake amount of the sample is small, it can be controlled with high accuracy by using a pump with such a small cylinder capacity. If the intake amount of the sample is large or at the time of intake of the rinse solution, the intake amount cannot be treated by a stroke of driving of the plunger, and it is necessary to repeat intake and discharge operations a plurality of times, which prolongs intake time of the sample or the rinse solution.
On the other hand, if a metering pump 5 of a large cylinder capacity is used, a large amount of the solution can be treated by a stroke of driving of the plunger, though accuracy of the intake amount decreases in taking in only a small amount of the sample.
In prior art, although it is possible to replace the metering pump with one with a suitable cylinder capacity according to the amount of the sample to be injected into the liquid chromatograph, it is troublesome to replace the plunger.